Preheat the oven to 400 degrees. Place the tortillas directly on the oven rack and cook for 8-10 minutes until crisp to create the tostada shells. You can also cook them on a baking sheet sprayed with a little cooking spray.
Heat the olive oil over medium-high heat. Add the pepper and onion. Cook for 4-5 minutes. Add the beef and taco seasoning. Cook for 6-8 minutes until almost fully cooked. If your taco seasoning calls for water, add that as well.
Heat the refried beans in a pan or the microwave. Build the tostadas by spreading a thin layer of beans on the corn tortillas. Then add the beef. Top with lettuce, tomatoes, cilantro, cheese, avocado, salsa, and any other toppings you choose.